The present invention provides a new pavement-marking sheet material of the general type described in U.S. Pat. No. 4,117,192. Such sheet material comprises a polymeric material that could be crosslinked to form an elastomer, but which is not crosslinked in the sheet material and thereby provides desired viscoelastic properties. A blend of this material with other polymeric materials and inorganic fillers has been found to provide properties that give long-lasting pavement markings--good conformability to a roadway surface, abrasion resistance, tensile and tear strength, etc.
Despite its well established utility, the described sheet material is deficient for some uses because asbestos fibers constitute a large proportion of the inorganic fillers in the sheet material. Asbestos fibers contribute importantly to the desired properties of the sheet material, but for toxicity reasons, use of such fibers has been foreclosed under many circumstances.
The new sheet material eliminates asbestos fibers, and does it without sacrificing desired properties of the sheet material. To the contrary, sheet material of the invention offers superior properties--superior abrasion resistance, higher tensile strength and modulus, and improved whiteness; and also the composition may be processed into sheet form more readily.
Briefly, these advantages are obtained with a composition that comprises 100 parts of non-crosslinked elastomer precursor.sup.1 : at least 5 parts of a thermoplastic reinforcing polymer which is dispersed in the elastomer as a separate phase (i.e., because of insolubility or immiscibility with the other polymeric ingredients) and softens at a temperature between about 75.degree. C.; and 200.degree. C.; and a particulate inorganic filler dispersed in the composition. This composition is processable on calendering rolls into a thin sheet material, generally between about 1/4 and 3 millimeters in thickness. FNT .sup.1 "Elastomer-precursor" is used herein to describe a polymer which can be crosslinked, vulcanized, or cured to form an elastomer. "Elastomer" is used to mean a material that can be stretched, to at least about twice its original dimensions without rupture and upon release of the stretching force rapidly returns to substantially its original dimensions.
The separate-phase nature of the reinforcing polymer is considered desirable, in that it is believed that the polymer becomes oriented during the calendering operation and reinforces the sheet material. Such a reinforcement is indicated by the fact that the tensile strength of the sheet material is significantly stronger in the downweb direction (i.e., in the direction of calendering) than in the crossweb, or transverse, direction.
Preferably the reinforcing polymer comprises polyolefin, especially polyethylene. An extender resin such as halogenated paraffin, which is soluble in the non-crosslinked elastomer precursor, is also preferably included in the composition. Also, platelet fillers, such as magnesium silicate, are preferred.
An earlier patent, U.S. Pat. No. 4,069,281, teaches a pavement-marking sheet material which includes low-molecular-weight polyethylene together with regenerated nitrile rubber and chlorobutadiene rubber (neoprene). However, when sheet material as described in this patent is made and calendered, no orientation or downweb reinforcement is detected. The sheet material is as strong in the crossweb direction as it is in the downweb direction. Apparently the low-molecular-weight polyethylene does not form an oriented reinforcement in the manner of the reinforcing polymer in sheet material of the invention. The prior art composition is low in tensile and tear strength, especially at elevated temperatures such as may be experienced in summer-time application of pavement markings; and it is difficult to process the composition in an internal mixer and calender it. Also, the sheet material is quite soft, making it more susceptible to dirt pick up, and glass microspheres tend to be pushed into the sheet material by road traffic, whereupon the microspheres are embedded and reflection is reduced.